optXXX() should return default value instead of throwing a RuntimeException

[h-cheon]

For example, the description of optBoolean is

     * Get an optional boolean associated with a key.
     * It returns the defaultValue if there is no such key, or if it is not
     * a Boolean or the String "true" or "false" (case insensitive).

But actually it returns the defaultValue only when the key is not in the json, and throws a RuntimeException in other cases.

    public boolean optBoolean(String key, boolean defaultValue) {
        Object o = opt(key);
        if (o == null) {
        	return defaultValue;
        } else {
        	try {
        	    return doGetBoolean(key, o);
        	} catch (JSONException ex) {
        		throw new RuntimeException(ex);
        	}
        }
    }